In recent years, hybrid vehicles and electric vehicles are gaining attention as environmentally friendly vehicles. A hybrid vehicle has, as a source of motive force, a conventional engine and in addition thereto a motor driven by a direct current power supply via an inverter. More specifically, the engine is driven to obtain a source of motive force and the direct current power supply also provides direct current voltage which is in turn converted by the inverter to alternate current voltage employed to rotate the motor to obtain a source of motive force.
An electric vehicle is a vehicle having as a source of motive force a motor driven by a direct current power supply via an inverter.
For hybrid vehicles and electric vehicles, a vehicle has been studied that drives right and left drive wheel units independently by individual motors or similar driving sources to drive wheel units independently. (See for example Japanese Patent Laying-Open Nos. 2001-28804, 2004-328991, 2001-78303, 2005-119647, 2-133005, 2004-175313, and 4-145810). It can provide enhanced driving force in comparison with an electric vehicle mounting only a single motor as a driving source, and also implement a delicate operation satisfying a request of a driver of a four wheel drive vehicle.
For example, Japanese Patent Laying-Open No. 2001-28804 discloses an electric vehicle having mounted therein a plurality of induction motors coupled with right and left drive wheel units to drive the drive wheel units, respectively, independently. Originally, an inverter is connected for each induction motor. According to the publication, such inverters are integrated into a single inverter to reduce the vehicle's weight and cost.
Herein if the right and left induction motors are driven by the common inverter, and the vehicle turns and has the right and left induction motors rotating at different speeds, the difference in rotational speed causes a difference between torques output, and the vehicle's cornering ability is impaired.
More specifically, an induction motor has a characteristic providing an output torque varying with slip velocity, which corresponds to a rotor's rotational speed minus a drive current's rotational speed. Thus when a vehicle turns, it has an outer wheel unit rotating at a speed higher than an inner wheel unit, and accordingly, the outer wheel unit's slip velocity becomes smaller than the inner wheel unit's slip velocity, and an induction motor that drives the inner wheel unit will thus output a larger torque than that which drives the outer wheel unit. As a result the vehicle's cornering ability is impaired.
Accordingly, Japanese Patent Laying-Open No. 2001-28804 adopts a configuration that controls slip velocity in accordance with a difference in rotational speed between the right and left induction motors to allow the induction motors to generate equal torques. Thus, when the vehicle is turning, the induction motor rotating at low speed can output a torque without increasing it. The vehicle can thus turn effectively smoothly.
Furthermore, Japanese Patent Laying-Open No. 2001-28804 describes that when the vehicle is traveling straight forward the right and left induction motors are compared in rotational speed, and when the induction motors are controlled in a powering mode the smaller rotational speed is set as a reference rotational speed, and when the induction motors are controlled in a regeneration mode, the larger rotational speed is set as a reference rotational speed, and in this way, vector control is performed. According to this, when the vehicle is traveling straight forward, and if it has a wheel unit slipping or skidding, a torque that is allocated to the slipping or skidding wheel unit can be reduced, and the motors can be controlled to eliminate slipping or skidding.
According to Japanese Patent Laying-Open No. 2001-28804, when the vehicle is traveling straight forward, the vector control can be performed to operate the induction motors in a range allowing the motors to be driven highly efficiently. However, when right and left drive motors rotate at the same speed, then the right and left induction motors will be controlled to be driven to output a target torque calculated from an accelerator pedal position, a shift position and the vehicle's speed, that is halved. As seen from the individual induction motors, this does not necessarily drive them highly efficiently. More specifically, a motor has a characteristic in efficiency that generally varies with the torque output and the number of revolutions. However, the torque output is balanced constantly at the right and left induction motors regardless of the torque output and the number of revolutions, and the induction motors may be driven less efficiently.
Accordingly, if a single inverter is used in driving a plurality of induction motors, reflecting the individual induction motors' respective drive efficiencies in allocating among the induction motors a driving force output from the induction motors would contribute to providing a vehicle reduced in weight and cost and also allow the vehicle to have the motors driven more efficiently as seen in total.
Furthermore, when a vehicle is turning, allocating a driving force output from the induction motors among the induction motors in accordance with in which direction the vehicle is turning, would also be effective in ensuring that the vehicle can travel safely.
The present invention has been made to overcome such disadvantage, and it contemplates a driving force control apparatus for a vehicle that drives wheel units independently, that can accomplish high drive efficiency and high traveling stability.